The overall biliary excretion of anionic drugs involves at least hepatic uptake and secretion. In many cases intrahepatic metabolism of the agent occurs prior to biliary secretion. At this time the kinetics of each component of the biliary excretory process are not known, nor is data available concerning the pharmacokinetic relationships between each of the components. The purpose of these studies is to obtain a clear understanding of each of the components of biliary excretion for a prototype organic anion, iopanoic acid, a widely used oral cholecystographic agent. Both in vivo and in vitro techniques will be used throughout the studies. It is anticipated that the data obtained from this project will form the basis for future investigations on drug interactions between therapeutic, as well as toxicological, agents.